I'm not certain your girlfriend would notice the change, but she might. Why not give it a shot?
It's certainly easier to wash more plain underwear - my boyfriend might not care if his undies get discolored, but I do.
He just started wearing boxer-briefs by some sportswear company, and the very fitted ones look absolutely delicious on him. I'm ready to pounce every time he wanders around his apartment in them. He doesn't have much of a butt, but those make what little he does have look awesome and irresistible. He also wears boxers. I don't really care about color, but he looks good in black, grey, and navy blue anything.
Can't say I'm a fan of plain briefs, but hot damn, I never knew boxer-briefs could look so good.
It's not a big deal at all, my preferences are really minor. Like, I would choose semi-loose boxer briefs over skintight briefs or super loose boxers, but none of those are complete turn-OFFs. Comfort is most important.
I like boxer briefs the most, doesn't really matter the color, just a solid color I think...No particular brand, because honestly, I know nothing about men underwear lol...
But wear whatever you feel comfortable in, that's the most important!